# Custom show for a struct that contains matrix
**URL:** https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816
**Category:** New to Julia
**Tags:** show
**Created:** [September 24, 2024, 4:23pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816 "2024-09-24T16:23:24Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@azoviktor](https://discourse.julialang.org/u/azoviktor)
#### Post date: [September 24, 2024, 4:23pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/1 "2024-09-24T16:23:24Z")
Suppose I have a struct
```julia
struct Box
size::Int
matrix::Matrix{ComplexF64}
end
```
and my aim is to implement a custom `show` method for a `Box` that will display its matrix in a nice rectangle form (not printing all the elements in a row). I would expect that I have to write smth like this:
```julia
function Base.show(io::IO, box::Box)
println(io, "Box of size $(box.size):"
println(io, " matrix:")
print(io, box.matrix)
end
```
but it prints all the matrix elements in one row. Using `display(box.matrix)` inside `show(io::IO, box::Box)` prints the matrix in a nice rectangle form, but it does it two times for some reason…
---
### Author:  [@eldee](https://discourse.julialang.org/u/eldee)
#### Post date: [September 24, 2024, 4:32pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/2 "2024-09-24T16:32:21Z")
Your code with `display` works fine for me (though it’s probably not ideal we don’t use `io` there, you could use `Base.show(io, "text/plain", box.matrix)` instead):
```julia
struct Box
size::Int
matrix::Matrix{ComplexF64}
end
function show(io::IO, box::Box)
println(io, "Box of size $(box.size):")
println(io, " matrix:")
display(box.matrix)
end
box = Box(3, rand(ComplexF64, 3, 3))
```
```julia-repl
julia> show(stdout, box)
Box of size 3:
matrix:
3×3 Matrix{ComplexF64}:
0.110489+0.490646im 0.216738+0.506696im 0.806984+0.535515im
0.569361+0.497763im 0.990934+0.791805im 0.344692+0.786712im
0.662557+0.48268im 0.550246+0.0305454im 0.0693276+0.143775im
```
I don’t know why would you get something different, but perhaps you’re somehow accidentally returning `box.matrix` from `show`?
```julia-repl
julia> function show(io::IO, box::Box)
println(io, "Box of size $(box.size):")
println(io, " matrix:")
display(box.matrix); box.matrix
end
show (generic function with 1 method)
julia> show(stdout, box)
Box of size 3:
matrix:
3×3 Matrix{ComplexF64}:
0.110489+0.490646im 0.216738+0.506696im 0.806984+0.535515im
0.569361+0.497763im 0.990934+0.791805im 0.344692+0.786712im
0.662557+0.48268im 0.550246+0.0305454im 0.0693276+0.143775im
3×3 Matrix{ComplexF64}:
0.110489+0.490646im 0.216738+0.506696im 0.806984+0.535515im
0.569361+0.497763im 0.990934+0.791805im 0.344692+0.786712im
0.662557+0.48268im 0.550246+0.0305454im 0.0693276+0.143775im
julia> show(stdout, box);
Box of size 3:
matrix:
3×3 Matrix{ComplexF64}:
0.110489+0.490646im 0.216738+0.506696im 0.806984+0.535515im
0.569361+0.497763im 0.990934+0.791805im 0.344692+0.786712im
0.662557+0.48268im 0.550246+0.0305454im 0.0693276+0.143775im
```
---
### Author:  [@azoviktor](https://discourse.julialang.org/u/azoviktor)
#### Post date: [September 24, 2024, 4:40pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/3 "2024-09-24T16:40:01Z")
Oh, right. Calling `show(stdout, box)` indeed prints it only once. And after printing just `box` in the Julia REPL prints it only once. However, in VScode editing window, when I press Shift+Enter to evaluate the command `box`, it prints it twice in the REPL for some reason:
```julia
Box of size 3:
matrix:
3×3 Matrix{ComplexF64}:
0.207312+0.832938im 0.656108+0.173641im 0.661501+0.384587im
0.983555+0.684514im 0.292562+0.617771im 0.0917733+0.096517im
0.0218214+0.730983im 0.986447+0.37221im 0.095009+0.715964im
3×3 Matrix{ComplexF64}:
0.207312+0.832938im 0.656108+0.173641im 0.661501+0.384587im
0.983555+0.684514im 0.292562+0.617771im 0.0917733+0.096517im
0.0218214+0.730983im 0.986447+0.37221im 0.095009+0.715964im
```
But maybe it is the problem with VScode…
---
### Author:  [@eldee](https://discourse.julialang.org/u/eldee)
#### Post date: [September 24, 2024, 4:50pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/4 "2024-09-24T16:50:47Z")
In my VS Code Shift + Enter on a line containing only the `box` variable prints the same as `println(box)`:
```julia-repl
Box(3, ComplexF64[0.17059151873830936 + 0.6467902853991271im 0.5516890187413159 + 0.16840124279428315im 0.04774010114262217 + 0.03149348949086228im; 0.16131392705103242 + 0.03850134950744866im 0.42382508938133245 + 0.2807228527058967im 0.32199193022264194 + 0.06329937630082594im; 0.6228329149783915 + 0.3768341650812741im 0.7527126699625651 + 0.16822047117462158im 0.36431680664443533 + 0.469961437362322im])
```
(and Shift + Enter on `show(stdout, box)` works as intended, with a single pretty print).
Perhaps you have installed other relevant extensions apart from the official Julia extension, or changed some settings of the latter, altering VS Code’s behaviour when displaying variables?
If you Shift+Enter on a line `box;`, do you still get a double print?
---
### Author:  [@azoviktor](https://discourse.julialang.org/u/azoviktor)
#### Post date: [September 24, 2024, 5:00pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/5 "2024-09-24T17:00:51Z")
I have only one Julia extension installed in VScode.
If I Shift+Enter on a line `box;` I get nothing printed. If I Shift+Enter on `box` gives me double pretty print, and Shift+Enter on `println(box)` gives single pretty print…
But what if you extend the `show` function from the `Base` module using `function Base.show(io::IO, box::Box)`?
---
### Author:  [@eldee](https://discourse.julialang.org/u/eldee)
#### Post date: [September 24, 2024, 5:09pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/6 "2024-09-24T17:09:07Z")
> [@azoviktor](#):
>
> But what if you extend the `show` function from the `Base` module using `function Base.show(io::IO, box::Box)`?
Oh yeah, that would make more sense. (I see you added the `Base.` in an edit, presumably after I started typing my first reply, so I always worked with the original (implicit) `Main.` version.)
With the `Base.show` version I do get the nice prints (using `box`, `show(stdout, box)` or `println(box)`), but still only one (both for scripts and Jupyter notebooks).
---
### Author:  [@azoviktor](https://discourse.julialang.org/u/azoviktor)
#### Post date: [September 24, 2024, 6:23pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/7 "2024-09-24T18:23:00Z")
It seems that this problem arises due to not specifying the IO in the `display` method. (I am using Apple M1 machine, if it is important…) Suppose I have the following code:
```julia
struct Box
size::Int
matrix::Matrix{ComplexF64}
end
function Base.show(io::IO, box::Box)
println(io, "Box of size $(box.size):")
println("111")
println(io, " matrix:")
println("222")
display(box.matrix)
end
box = Box(3, rand(ComplexF64, 2, 2))
```
Then the Shift+Enter command applied to `box` gives
```julia
Box of size 3:
111
matrix:
222
2×2 Matrix{ComplexF64}:
0.305076+0.347138im 0.456306+0.974372im
0.169632+0.881314im 0.435036+0.345573im
111
222
2×2 Matrix{ComplexF64}:
0.305076+0.347138im 0.456306+0.974372im
0.169632+0.881314im 0.435036+0.345573im
```
Would you know maybe another way than `display` to pretty print a matrix which takes IO as an argument?
---
### Author:  [@eldee](https://discourse.julialang.org/u/eldee)
#### Post date: [September 24, 2024, 6:25pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/8 "2024-09-24T18:25:23Z")
Well, you could try `Base.show(io, "text/plain", box.matrix)`.
---
### Author:  [@azoviktor](https://discourse.julialang.org/u/azoviktor)
#### Post date: [September 24, 2024, 6:30pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/9 "2024-09-24T18:30:46Z")
Ooh, awesome, it works well now. I should probably read about MIMEs in the Julia docs and what they are used for… Thank you!!
You actually suggested this in your first answer, but I overlooked it ☹
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [September 24, 2024, 6:55pm UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/10 "2024-09-24T18:55:47Z")
> [@azoviktor](#):
>
> Ooh, awesome, it works well now. I should probably read about MIMEs in the Julia docs and what they are used for… Thank you!!
In particular, the `show` method you are defining should probably be the 3-argument `show`, not the 2-argument `show`:
```julia
function Base.show(io::IO, mime::MIME"text/plain", box::Box)
...
end
```
and then call `show(io, mime, box.matrix)`. The reason is that you are defining a multi-line verbose representation designed for human consumption, whereas the 2-argument `show` should normally be a single-line and compatible with how you enter the `Box` in Julia (this is the default).
This is all described in the manual’s [chapter on custom pretty-printing](https://docs.julialang.org/en/v1/manual/types/#man-custom-pretty-printing). You might also want to check out the [new summary of output functions](https://github.com/JuliaLang/julia/blob/2943833bea8a8c05f47de1edc154bbb6888547a1/doc/src/manual/types.md#output-function-summary) that will appear in the next version of the manual.
---
### Author:  [@azoviktor](https://discourse.julialang.org/u/azoviktor)
#### Post date: [September 25, 2024, 9:51am UTC](https://discourse.julialang.org/t/custom-show-for-a-struct-that-contains-matrix/119816/11 "2024-09-25T09:51:23Z")
Thank you!